1. Field of the Invention
The present invention relates to an apparatus for processing a digital audio signal and, more particularly, to an apparatus adapted for use in recording, reproducing and transmitting an audio signal or the like with a high quality.
2. Description of the Prior Art
There is proposed a method of compressing digital audio data by the use of a technique termed adaptive predictive coding (APC). Such method is capable of recording audio data at a high density without deteriorating the signal-to-noise ratio or articulation thereof. And, a conventional signal processing apparatus contrived to perform such a recording operation is disclosed in, for example, U.S. Pat. No. 4,797,902 or U.S. Pat. No. 5,070,515 by the present applicant.
The apparatus cited above fundamentally comprises a predictive error generating circuit for dividing input digital data into blocks each composed of a predetermined number of words along the time base, then processing the data of each block to acquire a plurality of different frequency characteristics, and generating predictive error data thereof; a selecting circuit for selecting, out of such predictive error data obtained from the predictive error generating circuit, the data having the smallest one of maximum absolute values; a bit compressing circuit for compressing the bits of the selected predictive error data; a correcting circuit for processing the quantization error caused at the time of bit compression with predetermined frequency characteristics and feeding the processed error back to the bit compressing circuit; and a frequency characteristic control circuit for controlling the frequency characteristic of the correcting circuit in such a manner as to conform the same with the frequency characteristic optimal for obtaining the predictive error data having the smallest one of the maximum absolute values at the respective frequency characteristics.
For example, one block is composed of 28 words each consisting of 16-bit input audio digital data. The data in each block are processed with four different frequency characteristics, and the maximum predictive errors (absolute values) obtained by the processes with the respective characteristics are detected. And, the frequency characteristic relative to generation of the smallest value out of such four maximum predictive errors is selected as a characteristic for optimizing the signal-to-noise ratio.
The bits of the predictive error data produced as a result of the process with such selected frequency characteristic are compressed (requantized) by selection of, for example, 4 significant bits out of 16 bits. The requantization error resulting from the bit compression is processed with the same frequency characteristic as that used in the predictive error generating circuit and then is fed back to the bit compressing circuit, whereby the spectrum of the requantization noise is turned into white noise differently from the spectrum of the input audio data.
By adjusting the frequency characteristic of the requantization error thus fed back, it is rendered possible to distribute much of the requantization noise in a frequency band where the energy level of the audio data is high, so that the signal-to-requantization noise ratio (SNR) can be improved by the auditory masking effect.
The process of improving the auditory SNR by shaping the spectrum of the requantization noise in a manner to conform with the spectrum of the input audio data is the fundamental principle of the cited known apparatus. In strictly optimizing the relationship of mutual correspondence between the two spectra, precise analysis of the input signal becomes necessary to consequently require a huge amount of calculations and complicated noise shaping.
Therefore, the conventional apparatus is so contrived that, under an assumed condition where the frequency characteristic in the process of generating a predictive error corresponds substantially to the spectrum of the input signal, the frequency characteristic of the requantization error correcting circuit is set to have a relationship of 1:1 to the frequency characteristic of the predictive error generating circuit.
In such a constitution, however, there arises a problem that when the input audio signal is an intense attack sound of a piano or the like, the signal-to-noise ratio may be deteriorated on the contrary to consequently cause a noise.